1. Field of the Invention
The present invention relates to a turbine rotor formed of different materials welded together and a steam turbine including the turbine rotor.
2. Description of the Related Art
For most of high-temperature parts in thermal power generation facilities, ferritic heat-resistant steels excellent in manufacturability and economic efficiency have been used. A steam turbine of such a conventional thermal power generation facility is generally under a steam temperature condition on order of 600° C. or lower, and therefore, its major components such as a turbine rotor and moving blades are made of ferritic heat-resistant steel.
However, in recent years, improvement in efficiency of thermal power generation facilities have been actively promoted from a viewpoint of environmental protection, and accordingly, steam turbines utilizing high-temperature steam at about 600° C. are operated. Such a steam turbine includes components whose necessary characteristics cannot be satisfied by characteristics of the ferritic heat-resistant steel, and therefore, these components are sometimes made of a heat-resistant alloy or austenitic heat-resistant steel more excellent in high-temperature resistance.
For example, JP-A 7-247806(KOKAI), JP-A 2000-282808(KOKAI), and Japanese Patent Publication No. 3095745 (JP-B2) disclose arts to construct a steam turbine power generation facility with the minimum use of an austenitic material for a steam turbine utilizing high-temperature steam at 650° C. or higher. For example, in the steam turbine power generation facility described in JP-A 2000-282808(KOKAI), a superhigh-pressure turbine, a high-pressure turbine, an intermediate-pressure turbine, a low-pressure turbine, a second low-pressure turbine, and a generator are uniaxially connected, and the super high-pressure turbine and the high-pressure turbine are assembled in the same outer casing and thus are independent of the others.
Further, in view of global environmental protection, a need for still higher efficiency enabling a reduction in emissions of CO2, SOx, and NOx is currently increasing. One of the most effective measures to enhance plant thermal efficiency in a thermal power generation facility is to increase steam temperature, and the development of a steam turbine utilizing steam whose temperature is on order of 700° C. is under consideration.
Further, for example, JP-A 2004-353603(KOKAI) discloses an art to cool turbine components by cooling steam in order to cope with the aforesaid increase in the steam temperature.
For example, in the development of a steam turbine to which steam at a temperature of 630° C. or higher is introduced, there are many problems to be solved, in particular, regarding how strength of turbine components can be ensured. In thermal power generation facilities, improved heat-resistant steel has been conventionally used for turbine components such as a turbine rotor, nozzles, moving blades, a nozzle box (steam chamber), and a steam supply pipe included in a steam turbine, but when the temperature of reheated steam becomes 630° C. or higher, it is difficult to maintain high level of strength guarantee of the turbine components.
Under such circumstances, there is a demand for realizing a new art that is capable of maintaining high level of strength guarantee of turbine components in a steam turbine even when conventional improved heat-resistant steel is used as it is for the turbine components. One prospective new art to realize this is to use cooling steam for cooling the aforesaid turbine components. However, to cool, for example, a turbine rotor and a casing by the cooling steam in order to use the conventional material for portions corresponding to and after a first-stage turbine, a required amount of the cooling steam amounts to several % of an amount of main steam. Moreover, since the cooling steam flows into a channel portion, there arises a problem of deterioration in internal efficiency of a turbine itself in accordance with deterioration in blade cascade performance.
In a case where the high-temperature parts and the low-temperature parts are joined by welding or the like, the former being made of a Ni-based alloy such as Inco625, Inco617, and Inco713 (manufactured by Inco Limited) or austenitic steel such as SUS310, all of which are materials excellent in strength under high temperature and having steam oxidation resistance, and the latter being made of ferritic steel, new 12Cr steel, advanced 12Cr steel, 12Cr steel, or CrMoV steel, there occurs a problem of thermal stress generated in welded portions. Specifically, since a coefficient of linear expansion of a Ni-based alloy or austenitic steel used for the high-temperature parts is larger than a coefficient of linear expansion of ferritic steel or the like used for the low-temperature parts, a large thermal stress is generated in the welded portions due to a difference in expansion, which may possibly break a portion near the welded portions.